1. Field of the Invention
This invention relates to an image signal processing method, and more particularly, to such image signal processing method for producing interpolated signal values in an image signal.
2. Description of the Prior Art
The most high-resolution image signal that can be got from broadcast services in Japan is called “full-high-vision” which is constructed by horizontal 1920 pixels and vertical 1080 pixels. On the other hand, an advanced image signal system called “4K” which is constructed by double number of horizontal pixels and double number of vertical pixels compared with “full-high-vision” has been developed recently. In addition, display apparatuses for image signals of “4K” are already available. Thereby, some methods that convert the image signal of “full-high-vision” to the image signal of “4K” by interpolating pixels are proposed.
Conventional methods for providing interpolated pixel signal values in an image signal are well known as the bi-linear interpolating methods or the bi-cubic interpolating methods. Furthermore, methods that produce interpolated pixel signal values in an image signal by a discrete cosine transformation and an inverse discrete cosine transformation are disclosed in U.S. Pat. No. 5,168,375 issued Dec. 1, 1992 to M. Reisch et al. and U.S. Pat. No. 7,139,443 issued Nov. 22, 2006 to N. Ozawa.
The methods disclosed in those patents are accomplished by extracting a block of pixel signal values from an image signal, providing a set of frequency coefficients by a discrete cosine transformation with the block of pixel signal values, and providing interpolated pixel signal values by an inverse discrete cosine transformation, decreasing the sampling interval from that of original sampling locations, with the set of frequency coefficients. Since the set of frequency coefficients generated with the original pixel signal values is used for the inverse discrete cosine transformation, the frequency content of the image signal is not affected by the interpolated pixel signal values.
In addition, it is disclosed in U.S. Pat. No. 5,168,375 that the filtering function for an image signal produced by the inverse discrete cosine transformation is accomplished by modifying the set of frequency coefficients. Thus, the filtering function is accomplished by a simplified process when interpolated pixel signal values are produced by the discrete cosine transformation and the inverse discrete cosine transformation. On the other hand, since the filtering function in the spatial domain is accomplished by convolving a block of pixel signal values with a filter kernel, the procedure of the filtering function in the spatial domain is complicated.
Moreover, it is disclosed in U.S. Pat. No. 7,139,443 that an interpolated pixel signal value produced by the discrete cosine transformation and the inverse discrete cosine transformation is provided by a linear combination of each pixel signal value in a block of pixel signal values. By applying these methods, a procedure producing interpolated signal values becomes more simplified.
Meanwhile, most of image capturing apparatuses, not only video cameras and digital still cameras for consumer but also television cameras for broadcast, are fabricated with solid-state imaging devices recently. In an image capturing apparatus employing a solid-state imaging device, an image signal captured by pixels arranged on the imaging area of the solid-state imaging device is converted in an image signal format corresponding to a display apparatus used. Hence, when a television camera for the image signal of “full-high-vision” is fabricated by a solid-state imaging device comprising horizontal 1920 pixels and vertical 1080 pixels, each pixel of the solid-state imaging device corresponds to each pixel of the display apparatus for the image signal of “full-high-vision”.
Therefore, the conversion of the image signal of “full-high vision” to the image signal of “4K” corresponds to dividing each pixel of the solid-state imaging device in a half of the original pixels in the horizontal direction and the vertical direction. Then, when the image signal of “4K” is produced by interpolating the image signal of “full-high vision” provided by a solid-state imaging device, the image signal of “4K” should be compensated by the frequency response caused by dividing pixels of the solid-state imaging device. However, a filtering characteristic suitable to compensate the frequency response caused by dividing pixels of the solid-state imaging device is not disclosed in the prior art.